Check that input variables to melt are appropriate.
If id.vars or measure.vars are missing,
melt_check will do its
best to impute them. If you only supply one of id.vars and measure.vars,
melt will assume the remainder of the variables in the data set belong to
the other. If you supply neither, melt will assume discrete variables are
id variables and all other are measured.
melt_check(data, id.vars, measure.vars, variable.name, value.name)
vector of identifying variable names or indexes
vector of Measured variable names or indexes
name of variable used to store measured variable names
name of variable used to store values
a list giving id and measure variables names.